Semiconductor laser device

ABSTRACT

Disclosed is a nitride based III-V group compound semiconductor laser device of ridge waveguide type with an oscillation wavelength of about 410 nm which has a low driving voltage, a high half-width value θ //  of a FFP in a direction horizontal to a hetero interface, and a high kink level (i.e., good light output-injected current characteristics over the high-output range). This laser device is similar in structure to the related-art semiconductor laser device except for the current constricting layer formed in a ridge. It has a stacked film composed of an SiO 2  film (600 Å thick) and an amorphous Si film (300 Å thick) which are formed on the SiO 2  film by vapor deposition. The stacked film covers both sides of the ridge and a p-AlGaN cladding layer extending sideward from the base of the ridge. The SiO 2  film and Si film have respective thicknesses which are established such that the absorption coefficient of fundamental horizontal lateral mode is larger than the absorption coefficient of primary horizontal lateral mode. This structure results in a higher kink level, while suppressing the high-order horizontal lateral mode, a larger effective refractive index difference Δn, and a larger value of θ //  without the necessity for reducing the ridge width.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor laser device ofridge waveguide type. More particularly, the present invention relatesto a semiconductor laser device of ridge waveguide type which has adesirably controlled half-width value θ_(//) of a far field pattern(FFP) in a direction horizontal to a hetero interface, exhibits goodlaser characteristics during high-output operation and merely requires alow driving voltage.

[0002] Semiconductor laser devices of ridge waveguide type, includingthose which are based on GaAs or InP for long wavelengths and a nitridebased III-V group compound for short wavelengths, find use in a variousapplication areas because they are easy to manufacture.

[0003] The semiconductor laser device of ridge waveguide type belongs tothe category of index guided device. It has an upper portion of an uppercladding layer and a contact layer, both resembling a striped-shapedridge. The ridge is formed such that an insulating film covers bothsides of the ridge and the upper cladding layer extending sideward fromthe base of the ridge. This insulating film functions as a layer toconstrict electric current and provides an effective refractive indexdifference in the lateral direction for mode control.

[0004] An explanation is given below, with reference to FIG. 11, of thestructure of a related-art nitride based III-V group compoundsemiconductor laser device of ridge waveguide type which emits lightwith a wavelength of about 410 nm. This laser device is referred to as“nitride based semiconductor laser device” hereinafter.

[0005]FIG. 11 shows a related-art nitride based semiconductor laserdevice of ridge waveguide type 10 has basically a stacked structure inwhich a plularity of layers are stacked on a sapphire substrate 12. Theplularity of layers stacked on the sapphire substrate 12 are a laterallygrown GaN layer 14, an n-GaN contact layer 16, an n-AlGaN cladding layer18, an active layer 20, a p-AlGaN cladding layer 22, and a p-GaN contactlayer 24.

[0006] In the stacked structure, the upper portion of the p-AlGaNcladding layer 22 and the p-GaN contact layer 24 are formed as astriped-shaped ridge 26. A mesa structure extending in the samedirection as the ridge 26 is formed by the upper portion of the n-GaNcontact layer 16, the n-AlGaN cladding layer 18, the active layer 20,and the remaining portion 22 a of the p-AlGaN cladding layer 22.

[0007] The ridge 26 has a width (W) of about 1.7 μm. The remainingportion 22 a of the p-AlGaN cladding layer 22 which extends sidewardfrom the base of the ridge 26 has a thickness (T) of about 0.17 μm.

[0008] An insulating film 28 of SiO₂ (about 2000 Å thick) is formed onboth sides of the ridge 26, the side of the mesa structure above thep-AlGaN cladding layer 22 extending sideward from the base of the ridge26, and the n-AlGaN contact layer 16.

[0009] On the insulating film 28 is formed a p-side electrode 30, whichis in contact with the p-GaN contact layer 24 through a window in theinsulating film 28. On the n-GaN contact layer 16 is formed an n-sideelectrode 32.

[0010] The nitride based semiconductor laser device of ridge waveguidetype 10 mentioned above is considered as a highly efficient one becausethe insulating film 28 covering both sides of the ridge 26 istransparent to the emitted laser beam with little waveguide loss and thethreshold current is small.

[0011] In the meantime, as its application areas expand, the nitridebased semiconductor laser device of ridge waveguide type is required tohave a higher kink level so that it maintains good characteristicproperty for light output vs. injected current throughout the region upto the high-output level. It is also required to have a largerhalf-width value θ_(//) of a far field pattern (FFP) in a directionhorizontal to the hetero interface.

[0012] For example, in the case where the nitride based semiconductorlaser device is used as a light source of an optical pickup, it isrequired to have a larger half-width value θ_(//).

[0013] The results of the present inventors' investigation revealed thatthe value of θ_(//) is related closely with the difference (Δn) ofeffective refractive index of the ridge waveguide, as shown in FIG. 12.In order to obtain a larger value of θ_(//), it is necessary to have alarger value of Δn. Incidentally, the difference (Δn) of effectiverefractive index of the ridge waveguide is defined as n_(eff1)-n_(eff2)or a difference between n_(eff1) which is the effective refractive indexof the ridge for the oscillation wavelength and n_(eff2) which is theeffective refractive index of the ridge's side, as shown in FIG. 11.Closed and open circles in FIG. 12 denote the values obtained byexperiments.

[0014] Unfortunately, any attempt to increase the value of Δn ends upwith a narrow cutoff ridge width of high-order horizontal lateral mode.The cutoff ridge width of high-order horizontal lateral mode is definedas a ridge width which gives rise to no high-order horizontal lateralmode. When the ridge width is larger than the cutoff ridge width, thehorizontal lateral mode tends to shift from the fundamental mode to theprimary high-order mode at the time of laser oscillation.

[0015] When a hybrid mode consisting of the fundamental horizontallateral mode and the high-order horizontal lateral mode occurs, a kinkoccurs in the light output-injected current characteristics, as shown inFIG. 13. The result is a deterioration in the laser characteristics atthe time of high-output operation.

[0016] The foregoing holds true particular for the nitride basedsemiconductor laser device of ridge wave-guide type, which has a smallvalue of Δn and a short oscillation wavelength and hence has a narrowcutoff ridge width of high-order horizontal lateral mode, as shown inFIG. 14. FIG. 14 is a graph showing the relation between the value of Δnand the cutoff ridge width in the case where the GaN layer has arefractive index of 2.504 and an oscillation wavelength (λ) of 400 nm.Δn stands for the difference between the effective refractive index ofthe ridge and the effective refractive index of the ridge's side. Forexample, if the value of Δn is 0.005 to 0.01, the ridge width should bereduced to about 1 μm so that the ridge width is smaller than the cutoffridge width.

[0017] As mentioned above, any attempt to increase the value of Δn,thereby increasing the value of θ_(//), ends up with a decreased cutoffridge width, which leads to a deterioration in laser characteristics atthe time of high-output operation. In other words, there is a trade-offfor ridge width between the value of θ_(//) and the lasercharacteristics at the time of high-output operation.

[0018] Moreover, the nitride based semiconductor laser device of ridgewaveguide type has found an increasing use in the area of portablemachines. The one for this purpose is required to have a lower drivevoltage. One way to reduce the drive voltage is to increase the ridgewidth so that the contact area between the contact layer and the p-sideelectrode is increased. However, this suffers the disadvantage that theridge width exceeds the cutoff ridge width, resulting in a deteriorationin the laser characteristics at the time of high-output operation. Inother words, there is a trade-off for the ridge width between thereduced drive voltage and the improved laser characteristics at the timeof high-output operation.

[0019] The foregoing indicates that reducing the ridge width, therebyimproving the laser characteristics at the time of high-outputoperation, contradicts increasing the value of θ_(//) and decreasing thedrive voltage.

[0020] As mentioned above, the related-art nitride based semiconductorlaser device poses several problems. That is, it does not permit theridge width to be decreased appreciably in order to keep its drivevoltage low. Also, it has a ridge width lager than its cutoff ridgewidth, which prevents the kink level from being raised to a desired highlevel in the light output-injected current characteristics. The resultis that the value of Δn is small and the value of θ_(//) is also small.

[0021] The foregoing is applicable not only to nitride basedsemiconductor laser devices but also to any semiconductor laser devices(such as GaAs and InP) of ridge waveguide type for longer wavelengths.

SUMMARY OF THE INVENTION

[0022] It is an object of the present invention to provide asemiconductor laser device of ridge waveguide type which has a low drivevoltage, a large value of θ_(//), and a high kink level or good lightoutput-injected current characteristics up to a high output range.

[0023] The present inventors carried out extensive investigations insearch of a semiconductor laser device which has a large value of Δn anda large value of θ_(//) and keeps good light output-injected currentcharacteristics up to a high output range, without the necessity ofreducing the ridge width to keep the drive voltage low. As the result,it was found that a difference in absorption coefficient occurs betweenthe fundamental horizontal lateral mode and the primary horizontallateral mode, as shown in FIG. 15, if stacked layers are sequentiallyformed on both sides of the ridge, the stacked layers consisting of aninsulating film which does not absorb the laser beam appreciably, aninsulating film which is substantially transparent to the light of theoscillation wavelength, and a film which absorbs the laser beam.

[0024] It was also found that the above-mentioned phenomenon can beutilized to increase the kink level as high as practically acceptableand to increase the value of θ_(//) without reducing the ridge width.

[0025] In addition, the present inventors carried out a series ofexperiments on the combination of various insulating films andabsorption films. As the result, it was found that the insulating filmsand absorption films in stacked form (each film having a thicknessspecified in the present invention) suppress the high-order lateralmode. These findings led to the present invention.

[0026] The present invention to achieve the above-mentioned object isdirected to a semiconductor laser device of ridge waveguide typeincluding: a ridge formed in an upper portion of at least an uppercladding layer, wherein a stacked film composed of an insulating filmsubstantially transparent to the oscillation wavelength and anabsorption film formed on the insulating film which absorbs theoscillation wavelength, is formed on both sides of the ridge and on theupper cladding layer extending sideward from the base of the ridge, anelectrode film is electrically connected to the upper surface of theridge through a window in the stacked film, and the insulating film andthe absorption film have respective thicknesses such that the absorptioncoefficient of high-order horizontal lateral mode is larger than theabsorption coefficient of fundamental horizontal lateral mode.

[0027] According to the present invention, the ridge may be in any form(in plan view) without specific restrictions. It may be in a stripeform, taper form, or flare form.

[0028] The fact that the insulating film and the absorption film havefilm thicknesses which are respectively established such that theabsorption coefficient of high-order horizontal lateral mode is largerthan the absorption coefficient of fundamental horizontal lateral modesuppresses the high-order horizontal lateral mode and increases the kinklevel in the high output region without the necessity of reducing theridge width, and also increases the value of Δn and the value of θ_(//).

[0029] According to the present invention, the insulating film is notspecifically restricted in its kind so long as it is transparent to theoscillation wavelength, and the absorption film is not specificallyrestricted in its kind so long as it absorbs the oscillation wavelength.

[0030] “Insulating film substantially transparent to the oscillationwavelength” means a film whose absorption edge is shorter than theoscillation wavelength. “Absorption film” means a film whose absorptionedge is longer than the oscillation wavelength.

[0031] According to the present invention, the insulating film may beany of an SiO₂ film, Al₂O₃ film, AlN film, SiN_(x) film, Ta₂O₅ film, andZrO₂ film, and the absorption film may be an Si film which is usually anamorphous Si film.

[0032] The insulating film such as SiO₂ film, Si film, and ZrO₂ filmshould preferably be formed by vapor deposition.

[0033] The semiconductor laser device of the present invention has aresonator structure of nitride based III-V group compound semiconductorlayer formed on a substrate and also has an AlGaN cladding layer (as anupper cladding layer) whose upper portion is formed as the ridge. Inthis semiconductor laser device, the insulating film (SiO₂ film) has athickness of 200 Å to 800 Å and the absorption film (Si film) has athickness of 50 Å and above.

[0034] The thickness of the Si film, which is 50 Å and above, has beenestablished from the following simulation. The simulation was run with amodel in which the insulating film consists of an SiO₂ film with aconstant thickness of 600 Å and an Si film with a varied thickness, asshown in FIG. 16A. Calculations by this simulation predicted the changein absorption coefficient of the fundamental horizontal lateral mode andthe primary horizontal lateral mode, as shown in FIG. 16B. The curve (1)represents the absorption coefficient of fundamental horizontal lateralmode, and the curve (2) represents the absorption coefficient of primaryhorizontal lateral mode.

[0035] Since it is desirable that the absorption coefficient a ofprimary horizontal lateral mode be at least 10 cm⁻¹, the thickness ofthe Si film should be equal to or larger than 50 Å, preferably equal toor larger than 200 Å.

[0036] For desirable results, the thickness of the SiO₂ film as aninsulating film should be 400 Å to 800 Å and the thickness of the Sifilm as an absorption film should be 50 Å and above. For more desirableresults, the thickness of the SiO₂ film as an insulating film should be400 Å to 800 Å and the thickness of the Si film as an absorption filmshould be 200 Å and above.

[0037] If the thickness of the SiO₂ film exceeds 800 Å, there will be nodifference between the absorption coefficient of high-order horizontallateral mode and the coefficient of fundamental horizontal lateral mode,which results in a small value of Δn. By contrast, if the thickness ofthe SiO₂ film is not more than 400 Å, the absorption coefficient offundamental horizontal lateral mode is excessively small, which resultsin an increased threshold current.

[0038] The thickness of the ZrO₂ film as an insulating film should be200 Å to 1200 Å and the thickness of the Si film as an absorption filmshould be 50 Å and above. For desirable results, the thickness of theZrO₂ film as an insulating film should be 300 Å to 1100 Å and thethickness of the Si film as an absorption film should be 50 Å and above.For more desirable results, the thickness of the ZrO₂ film as aninsulating film should be 600 Å to 1100 Å and the thickness of the Sifilm as an absorption film should be 200 Å and above.

[0039] If the thickness of the ZrO₂ film exceeds 1200 Å, there will beno difference between the absorption coefficient of high-orderhorizontal lateral mode and the coefficient of fundamental horizontallateral mode, which results in a small value of Δn. By contrast, if thethickness of the ZrO₂ film is not more than 200 Å, the absorptioncoefficient of fundamental horizontal lateral mode is excessively small,which results in an increased threshold current.

[0040] Furthermore, the insulating film may be replaced by any of anAl₂O₃ film (200 Å to 1000 Å thick), an SiN_(x) film (200 Å to 1200 Åthick), an AlN film (200 Å to 1400 Å thick), a Ta₂O₅ film (200 Å to 1200Å thick), and a ZrO₂ film (200 Å to 1200 Å thick). The insulating filmis combined with an Si film with a thickness of 50 Å and above as anabsorption film to form the stacked film.

[0041] Alternatively, the stacked film may be a combination of a metalfilm as an absorption film and any of the following insulating films: anAl₂O₃ film (200 Å to 1000 Å thick), an SiN_(x) film (200 Å to 1200 Åthick), an AlN film (200 Å to 1400 Å thick), a Ta₂O₅ film (200 Å to 1200Å thick), and a ZrO₂ film (200 Å to 1000 Å thick) The stacked film mayalso be formed from an SiO₂ film (100 Å to 800 Å thick) or a ZrO₂ film(200 Å to 1000 Å thick) as an insulating film and a metal film as anabsorption film. The metal film may be formed from Ni, Pt, or Au with athickness of 10, 100, or 300 nm, respectively. The metal film mayfunction as an electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a sectional view showing the structure of essentialparts of a nitride based semiconductor laser device according toEmbodiment 1 and Example 1;

[0043]FIG. 2 is a graph showing the relation between the thickness of anSiO₂ film formed by vapor deposition and the absorption coefficients offundamental horizontal lateral mode and primary horizontal lateral modein the nitride based semiconductor laser device according to Embodiment1;

[0044]FIG. 3 is a graph showing the relation between the effectiverefractive index difference Δn and the kink level in both the nitridebased semiconductor laser device according to Embodiment 1 and arelated-art nitride based semiconductor laser device;

[0045]FIG. 4 is a graph showing the relation between the half-widthvalue θ_(//) of a far field pattern (FFP) in a direction horizontal to ahetero interface and the kink level in both the nitride basedsemiconductor laser device according to Embodiment 1 and the related-artnitride based semiconductor laser device;

[0046]FIG. 5 is a sectional view showing the structure of essentialparts of a nitride based semiconductor laser device according toEmbodiment 2 and Example 2;

[0047]FIG. 6 is a graph showing the relation between the thickness of anSiO₂ film formed by vapor deposition and the absorption coefficients offundamental horizontal lateral mode and primary horizontal lateral modein the nitride based semiconductor laser device according to Embodiment2;

[0048]FIG. 7 is a sectional view showing the structure of essentialparts of a nitride based semiconductor laser device according toEmbodiment 3 and Example 3;

[0049]FIG. 8 is a graph showing the relation between the thickness of aZrO₂ film formed by vapor deposition and the absorption coefficients offundamental horizontal lateral mode and primary horizontal lateral modein the nitride based semiconductor laser device according to Embodiment3;

[0050]FIG. 9 is a sectional view showing the structure of essentialparts of a nitride based semiconductor laser device according toEmbodiment 4 and Example 4;

[0051]FIG. 10 is a graph showing the relation between the thickness of aZrO₂ film formed by vapor deposition and the absorption coefficients offundamental horizontal lateral mode and primary horizontal lateral modein the nitride based semiconductor laser device according to Embodiment4;

[0052]FIG. 11 is a sectional view showing the structure of therelated-art nitride based semiconductor laser device;

[0053]FIG. 12 is a graph showing the relation between the value of Δnand the value of θ_(//) in a nitride based semiconductor laser device;

[0054]FIG. 13 is a graph showing the kink level in terms of lightoutput-injection current characteristics;

[0055]FIG. 14 is a graph showing the relation between the value of Δnand the cutoff ridge width in a nitride based semiconductor laserdevice;

[0056]FIG. 15 is a schematic diagram showing the absorption loss infundamental horizontal lateral mode and primary horizontal lateral mode;and

[0057]FIGS. 16A and 16B are graphs showing respectively the structure ofa stacked film and the relation between the thickness of an Si film andthe absorption coefficients of fundamental horizontal lateral mode andprimary horizontal lateral mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] The embodiments of the present invention will be describedspecifically in more detail with reference to the accompanying drawings.As an illustration for easy understanding of the present invention, thefollowing embodiments specify the film forming method, the compositionand thickness of the compound semiconductor layers, the ridge width, andthe process conditions. This illustration is not intended to restrictthe scope of the present invention.

[0059] Embodiment 1

[0060] The present invention is embodied in a nitride based III-V groupcompound semiconductor laser device (referred to as “nitride basedsemiconductor laser device” hereinafter), which consists of essentialparts shown in a sectional view in FIG. 1.

[0061] The nitride based semiconductor laser device consists of asubstrate and a resonator structure composed of a nitride based III-Vgroup compound semiconductor layer formed on the substrate. A ridge isformed in an upper layer of an upper cladding layer of AlGaN on thenitride based semiconductor laser device. The nitride basedsemiconductor laser device is similar in structure to the related-artnitride based semiconductor laser device 10 shown in FIG. 11, except forthe current constricting layer on the upper cladding layer 22 extendingsideward from the base of the ridge 26.

[0062] In this embodiment, the current constricting layer on the uppercladding layer 22 is a stacked film, formed on both sides of the ridge,composed of an SiO₂ film and an Si film. The SiO₂ film is substantiallytransparent to the light of the oscillation wavelength, and the Si filmabsorbs the light of the oscillation wavelength. Each of them has aspecific thickness to suppress the high-order lateral mode.

[0063] In this embodiment, the p-side electrode 30 is electricallyconnected to the p-GaN contact layer 24 on the ridge 26 through a windowin the stacked film.

[0064] The SiO₂ film and the Si film are formed sequentially by vapordeposition. The resulting stacked film has the window for the p-sideelectrode 30 formed by photolithography and reactive ion etching.

[0065] For the purpose of evaluation, various samples were prepared inwhich the thickness of the amorphous Si film 44 is fixed at 300 Å andthe thickness of the SiO₂ film 42 is varied. The samples were examinedfor change in the absorption coefficient of fundamental horizontallateral mode and the absorption coefficient of primary horizontallateral mode. The results are shown in FIG. 2.

[0066] Curve (1) in FIG. 2 represents the relation between the thicknessof the SiO₂ film 42 and the absorption coefficient of fundamentalhorizontal lateral mode. Curve (2) in FIG. 2 represents the relationbetween the thickness of the SiO₂ film 42 and the absorption coefficientof primary horizontal lateral mode.

[0067] This embodiment is characterized by the ridge 26 having the SiO₂film 42 and the amorphous Si film 44. This stacked structure keeps lowthe absorption coefficient of fundamental horizontal lateral mode asindicated by curve (1) in FIG. 2 and keeps high the absorptioncoefficient of primary horizontal lateral mode as indicated by curve (2)in FIG. 2. The result is an increased value of Δn with the ridge widthremaining unchanged.

[0068] This embodiment requires that the SiO₂ film 42 have a thicknessranging from 400 Å to 800 Å. With a thickness not more than 400 Å, theabsorption coefficient a of fundamental horizontal lateral mode is 15cm⁻¹ or more, resulting in an increased threshold current and adecreased light-emitting efficiency. With a thickness not less than 800Å, the absorption coefficient of primary horizontal lateral mode comesclose to the absorption coefficient of fundamental horizontal lateralmode, resulting in a decreased value of Δn.

[0069] In a modified embodiment, samples of a nitride basedsemiconductor laser device of related-art structure were prepared whichdiffer in that the insulating film is an SiO₂ deposited film (600 Åthick) and the absorption film is an amorphous Si deposited film (400 Åthick) and the upper cladding layer 22 has a varied thickness in itsparts extending sideward from the base of the ridge. Because of thisstructure, the samples vary in the effective refractive index difference(Δn) of the ridge waveguide. They were examined for relation between thevalue of Δn and the kink level.

[0070] It is noted from the results shown in FIG. 3 that the sampleshave higher values of Δn for the same kink level as compared with therelated-art semiconductor laser devices. For example, the samples havethe values of Δn at 0.009 and 0.0085 for the values of kink level at 60mW and 100 mW, respectively, whereas the related-art semiconductor laserdevices have the values of Δn at 0.0065 and 0.0045 for the values ofkink level at 60 mW and 100 mW, respectively. These small kink levelslead to a small value of θ_(//).

[0071] The samples mentioned above were examined for relation betweenthe kink level and the value of θ_(//). The results are shown in FIG. 4.It is noted from FIG. 4 that the samples have higher values of θ_(//)for the same kink level as compared with the related-art semiconductorlaser devices.

[0072] The foregoing suggests that the nitride based semiconductor laserdevice according to this embodiment has a low drive current, a high kinklevel, and a high value of θ_(//).

EXAMPLE 1

[0073] A concrete example of Embodiment 1 is illustrated by a sample ofa nitride based III-V group compound semiconductor laser device havingan oscillation wavelength of around 410 nm. This sample has the samestructure as the related-art semiconductor laser device 10 shown in FIG.11 except for the current constricting layer in the ridge 26. As in therelated-art semiconductor laser device 10, the width of the ridge 26 is1.7 μm and the thickness of the p-AlGaN cladding layer 22 extendingsideward from the base of the ridge 26 is 0.17 μm.

[0074] The sample in this example has a stacked film composed of an SiO₂film 42 (600 Å thick) and an amorphous Si film 44 (300 Å thick), whichare sequentially formed by vapor deposition. This stacked film coversthe sides of the ridge 26 and the p-AlGaN cladding layer 22 extendingsideward from the base of the ridge 26. The stacked film has a windowthrough which the p-side electrode 30 is electrically connected to thep-GaN contact layer 24.

[0075] Owing to the above-mentioned structure, the nitride basedsemiconductor laser device of this example has a kink level of 100 mWand a value of θ_(//) of 9.5°.

[0076] Embodiment 2

[0077] The second embodiment of the present invention is a nitride basedsemiconductor laser device whose essential parts are constructed asshown in a sectional view in FIG. 5.

[0078] The sample in this embodiment is similar in structure to that inEmbodiment 1 except for the current constricting layer in the ridge 26.The sample in this embodiment has a stacked film composed of an SiO₂film 46 as an insulating film and a p-side electrode 30 as an absorptionfilm, sequentially. As shown in FIG. 5, this stacked film covers thesides of the ridge 26 and the p-AlGaN cladding layer 22 extendingsideward from the base of the ridge 26.

[0079] For the purpose of evaluation, several samples were prepared inwhich the p-side electrode 30 has a fixed thickness (40 nm) and the SiO₂film 46 has a varied thickness. They were tested for change in theabsorption coefficient of fundamental horizontal lateral mode and theabsorption coefficient of primary horizontal lateral mode. The resultsare shown in FIG. 6.

[0080] Curve (1) in FIG. 6 represents the relation between the thicknessof the SiO₂ film 46 and the absorption coefficient of fundamentalhorizontal lateral mode. Curve (2) in FIG. 6 represents the relationbetween the thickness of the SiO₂ film 46 and the absorption coefficientof primary horizontal lateral mode. In both cases, the thickness of thep-side electrode 30 is fixed at 40 nm.

[0081] In this embodiment, the ridge 26 has a stacked film composed ofthe SiO₂ film 46 and the p-side electrode 30. This structure keeps theabsorption coefficient of fundamental horizontal lateral mode low asindicated by curve (1) and keeps the absorption coefficient of primaryhorizontal lateral mode high as indicated by curve (2). Therefore, thesamples have a larger value of Δn without change in the ridge width.

[0082] This embodiment requires that the SiO₂ film 46 have a thicknessranging from 100 Å to 800 Å. With a thickness not more than 100 Å, theabsorption coefficient a of fundamental horizontal lateral mode is 15cm⁻¹ or more, resulting in an increased threshold current and adecreased light-emitting efficiency. With a thickness not less than 800Å, the absorption coefficient of primary horizontal lateral mode comesclose to the absorption coefficient of fundamental horizontal lateralmode, resulting in a decreased value of Δn.

[0083] The foregoing suggests that the nitride based semiconductor laserdevice according to this embodiment has a low drive current, a high kinklevel, and a high value of θ_(//).

EXAMPLE 2

[0084] A concrete example of Embodiment 2 is illustrated by a sample ofa nitride based III-V group compound semiconductor laser device havingan oscillation wavelength of around 410 nm. This sample has the samestructure as the related-art semiconductor laser device 10 shown in FIG.11 except for the current constricting layer in the ridge 26. It isidentical with the related-art semiconductor laser device 10 in thewidth W of the ridge 26 and the thickness T of the p-AlGaN claddinglayer 22 extending sideward from the base of the ridge 26.

[0085] The sample in this example has a stacked film composed of an SiO₂film 46 (400 Å thick) and a p-side electrode 30 which is a metal film ofNi/Pt/Au with a thickness of 10/100/300 nm, respectively, which aresequentially formed by vapor deposition. This stacked film covers thesides of the ridge 26 and the p-AlGaN cladding layer 22 extendingsideward from the base of the ridge 26, as shown in FIG. 5. The SiO₂film 46 has a window through which the p-side electrode 30 iselectrically connected to the p-GaN contact layer 24.

[0086] Owing to the above-mentioned structure, the nitride basedcompound semiconductor laser device of this example has a kink level of80 mW and a value of θ_(//) of 9.8°.

[0087] Embodiment 3

[0088] The third embodiment of the present invention is a nitride basedsemiconductor laser device whose essential parts are constructed asshown in a sectional view in FIG. 7.

[0089] The sample in this embodiment is similar in structure to that inEmbodiment 1 except for the insulating film. The sample in thisembodiment has a stacked film composed of a ZrO₂ film 48 as aninsulating film and an amorphous Si film 44 (as an absorption film),both formed sequentially by vapor deposition. As shown in FIG. 7, thisstacked film covers the sides of the ridge 26 and the p-AlGaN claddinglayer 22 extending sideward from the base of the ridge 26.

[0090] For the purpose of evaluation, several samples were prepared inwhich the amorphous Si film 44 has a fixed thickness (300 Å) and theZrO₂ film 48 has a varied thickness. They were tested for change in theabsorption coefficient of fundamental horizontal lateral mode and theabsorption coefficient of primary horizontal lateral mode. The resultsare shown in FIG. 8.

[0091] Curve (1) in FIG. 8 represents the relation between the thicknessof the ZrO₂ film 48 and the absorption coefficient of fundamentalhorizontal lateral mode. Curve (2) in FIG. 8 represents the relationbetween the thickness of the ZrO₂ film 48 and the absorption coefficientof primary horizontal lateral mode. In both cases, the thickness of theamorphous Si film 44 is fixed at 300 Å.

[0092] In this embodiment, the ridge 26 has a stacked film composed ofthe ZrO₂ film 48 and the amorphous Si film 44. This structure keeps theabsorption coefficient of fundamental horizontal lateral mode low asindicated by curve (1) and keeps the absorption coefficient of primaryhorizontal lateral mode high as indicated by curve (2). Therefore, thesamples have a larger value of An without change in the ridge width.

[0093] This embodiment requires that the ZrO₂ film 48 have a thicknessranging from 600 Å to 1100 Å. With a thickness not more than 600 Å, theabsorption coefficient a of fundamental horizontal lateral mode is 15cm⁻¹ or more, resulting in an increased threshold current and adecreased light-emitting efficiency. With a thickness not less than 1100Å, the absorption coefficient of primary horizontal lateral mode comesclose to the absorption coefficient of fundamental horizontal lateralmode, resulting in a decreased value of Δn.

[0094] The foregoing suggests that the nitride based semiconductor laserdevice according to this embodiment has a low drive current, a high kinklevel, and a high value of θ_(//).

EXAMPLE 3

[0095] A concrete example of Embodiment 3 is illustrated by a sample ofa nitride based semiconductor laser device, which has the same structureas that in Example 1 except for the insulating film on the ridge 26.This insulating film is the ZrO₂ film 48 (800 ↑ thick) in place of theSiO₂ film 42 (600 Å thick).

[0096] Owing to the above-mentioned structure, the nitride basedsemiconductor laser device of this example has a kink level of 95 mW anda value of θ_(//) of 9.6°.

[0097] Embodiment 4

[0098] The fourth embodiment of the present invention is a nitride basedsemiconductor laser device whose essential parts are constructed asshown in a sectional view in FIG. 9.

[0099] The sample in this embodiment is similar in structure to that inEmbodiment 2 except for the insulating film formed on the ridge 26. Thesample in this embodiment has a stacked film composed of a ZrO₂ film 50as an insulating film and a p-side electrode 30 as an absorption film,sequentially. As shown in FIG. 9, this stacked film covers the sides ofthe ridge 26 and the p-AlGaN cladding layer 22 extending sideward fromthe base of the ridge 26.

[0100] For the purpose of evaluation, several samples were prepared inwhich the p-side electrode 30 has a fixed thickness (40 nm) and the ZrO₂film 50 has a varied thickness. They were tested for change in theabsorption coefficient of fundamental horizontal lateral mode and theabsorption coefficient of primary horizontal lateral mode. The resultsare shown in FIG. 10.

[0101] Curve (1) in FIG. 10 represents the relation between thethickness of the ZrO₂ film 50 and the absorption coefficient offundamental horizontal lateral mode. Curve (2) in FIG. 10 represents therelation between the thickness of the ZrO₂ film 50 and the absorptioncoefficient of primary horizontal lateral mode. In both cases, thethickness of the p-side electrode 30 is fixed at 40 nm.

[0102] In this embodiment, the ridge 26 has a stacked film composed ofthe ZrO₂ film 50 and the p-side electrode 30. This structure keeps theabsorption coefficient of fundamental horizontal lateral mode low asindicated by curve (1) and keeps the absorption coefficient of primaryhorizontal lateral mode high as indicated by curve (2). Therefore, thesamples have a larger value of Δn without change in the ridge width.

[0103] This embodiment requires that the ZrO₂ film 50 have a thicknessranging from 200 Å to 1000 Å. With a thickness not more than 200 Å, theabsorption coefficient a of fundamental horizontal lateral mode is 15cm⁻¹ or more, resulting in an increased threshold current and adecreased light-emitting efficiency. With a thickness not less than 1000Å, the absorption coefficient of primary horizontal lateral mode comesclose to the absorption coefficient of fundamental horizontal lateralmode, resulting in a decreased value of Δn.

[0104] The foregoing suggests that the nitride based semiconductor laserdevice according to this embodiment has a low drive current, a high kinklevel, and a high value of θ_(//).

EXAMPLE 4

[0105] A concrete example of Embodiment 2 is illustrated by a sample ofa nitride based III-V group compound semiconductor laser device havingan oscillation wavelength of around 410 nm. This sample has the samestructure as that in Example 2 except for the insulating film.

[0106] The sample in this example has a stacked film composed of a ZrO₂film 50 (600 Å thick) and a p-side electrode 30 which is a metal film ofNi/Pt/Au having a thickness of 10/100/300 nm, respectively, which aresequentially formed by vapor deposition. This stacked film covers thesides of the ridge 26 and the p-AlGaN cladding layer 22 extendingsideward from the base of the ridge 26, as shown in FIG. 9. The ZrO₂film 50 has a window through which the p-side electrode 30 iselectrically connected to the p-GaN contact layer 24.

[0107] Owing to the above-mentioned structure, the nitride basedsemiconductor laser device of this example has a kink level of 100 mWand a value of θ_(//) of 9.5°.

[0108] The above-mentioned Examples 1 to 4 demonstrate specific cases inwhich the thicknesses of the insulating film and the absorption film areadequate when the ridge 26 has the width W of 1.7 μm and the remainingportion of the p-AlGaN cladding layer 22 has a thickness of 0.17 μm. Theadequate film thicknesses vary depending on the shape and dimensions ofthe ridge.

[0109] The above-mentioned Examples 1 to 4 are not intended to restrictthe width of the ridge, the thickness of the cladding layer extendingsideward from the base of the ridge, and the thickness and kind of theinsulating film, so long as the semiconductor laser device isconstructed such that the absorption coefficient of primary mode islarger than that of fundamental mode.

[0110] According to the present invention, the nitride based III-V groupcompound semiconductor laser device of ridge waveguide type has astacked film composed of an insulating film substantially transparent tothe oscillation wavelength and an absorption film, on the insulatingfilm, which absorbs the oscillation wavelength, the stacked filmcovering the sides of the ridge and also covering the cladding layerextending sideward from the base of the ridge. The insulating film andthe absorption film have specific thicknesses respectively so that theabsorption coefficient of high-order horizontal lateral mode is largerthan the absorption coefficient of fundamental horizontal lateral mode.The above-mentioned unique structure results in a higher kink level, alarger value of Δn, and a larger value of θ_(//), while suppressing thehigh-order horizontal lateral mode, without the necessity of reducingthe width of the ridge.

[0111] The high kink level contributes to improved noisecharacteristics, and the larger value of Δn contributes to an increasedprocess margin.

[0112] Moreover, the high kink level implies that the resonator can havea more freely selected length and the ridge can have a larger width. Therelated-art structure restricts the resonance length because the kinklevel depends on the resonance length, and it also makes it necessary toreduce the ridge width to increase the kink level because the kink leveldepends on the ridge width. The result is a decrease in a drive voltageand an improvement in long-term reliability.

What is claimed is:
 1. A semiconductor laser device of ridge waveguidetype comprising: a ridge formed in an upper portion of at least an uppercladding layer, wherein a stacked film composed of an insulating filmsubstantially transparent to the oscillation wavelength and anabsorption film formed on the insulating film which absorbs theoscillation wavelength, is formed on both sides of the ridge and on theupper cladding layer extending sideward from the base of the ridge, anelectrode film is electrically connected to the upper surface of theridge through a window in the stacked film, and the insulating film andthe absorption film have respective thicknesses such that the absorptioncoefficient of high-order horizontal lateral mode is larger than theabsorption coefficient of fundamental horizontal lateral mode.
 2. Asemiconductor laser device as defined in claim 1, wherein the ridge isin any form of stripe, taper, or flare.
 3. A semiconductor laser deviceas defined in claim 1, wherein a resonator structure composed of nitridebased III-V group compound semiconductor layers is formed on thesubstrate, and a ridge is formed in an upper portion of an uppercladding layer composed of AlGaN.
 4. A semiconductor laser device asdefined in claim 1, wherein the insulating film is any of an SiO₂ film,Al₂O₃ film, AlN film, SiN_(x) film, Ta₂O₅ film, and ZrO₂ film, and theabsorption film is an Si film.
 5. A semiconductor laser device asdefined in claim 4, wherein the SiO₂ film as an insulating film has athickness of 200 Å to 800 Å and the Si film as an absorption film has athickness of 50 Å and above.
 6. A semiconductor laser device as definedin claim 4, wherein the insulating film is any of an Al₂O₃ film having athickness of 200 Å to 1000 Å, an SiN_(x) film having a thickness of 200Å to 1200 Å, an AlN film having a thickness of 200 Å to 1400 Å, a Ta₂O₅film having a thickness of 200 Å to 1200 Å, and a ZrO₂ film having athickness of 200 Å to 1200 Å, and the absorption film is an Si filmhaving a thickness of 50 Å and above.
 7. A semiconductor laser device asdefined in claim 1, wherein the insulating film is any of an SiO₂ filmhaving a thickness of 100 Å to 800 Å, an Al₂O₃ film having a thicknessof 100 Å to 800 Å, an SiN_(x) film having a thickness of 200 Å to 1000Å, an AlN film having a thickness of 200 Å to 1200 Å, a Ta₂O₅ filmhaving a thickness of 200 Å to 1000 Å, and a ZrO₂ film having athickness of 200 Å to 1000 Å, and the absorption film is a metal film.8. A semiconductor laser device as defined in claim 7, wherein the metalfilm as an absorption film functions as an electrode which iselectrically connected to an upper surface of the ridge through a windowin the insulating film.
 9. A semiconductor laser device as defined inclaim 8, wherein a resonator structure composed of nitride based III-Vgroup compound semiconductor layers is formed on the substrate, and aridge is formed in an upper layer of an upper cladding layer composed ofAlGaN.